Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as corn or sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose material, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. During the reduction of alkanoic acid, e.g., acetic acid, other compounds are often formed with ethanol or are formed in side reactions. For example, during hydrogenation, esters are produced that together with ethanol and/or water form azeotropes, which are difficult to separate. These impurities may limit the production of ethanol and may require expensive and complex purification trains to separate the impurities from the ethanol. Also, the hydrogenation of acetic acid typically yields ethanol and water along with small amounts of side reaction-generated impurities and/or by-products. At maximum theoretical conversion and selectivity, the crude ethanol product would comprise approximately 72 wt. % ethanol and 28 wt. % water. In order to form purified ethanol, much of the co-produced water must be removed from the crude ethanol composition. In addition, when conversion is incomplete, unreacted acid may remain in the crude ethanol product. It is typically desirable to remove this residual acetic acid from the crude ethanol product to yield purified ethanol.
Some processes for integrating acetic acid production and hydrogenation have been proposed in literature. Generally, acetic acid production produces glacial acetic acid that has less than 1500 wppm water.
For example, U.S. Pat. No. 7,884,253 discloses methods and apparatuses for selectively producing ethanol from syngas. The syngas is derived from cellulosic biomass (or other sources) and can be catalytically converted into methanol, which in turn can be catalytically converted into acetic acid or acetates. The ethanoic acid product may be removed from the reactor by withdrawing liquid reaction composition and separating the ethanoic acid product by one or more flash and/or fractional distillation stages from the other components of the liquid reaction composition such as iridium catalyst, ruthenium and/or osmium and/or indium promoter, methyl iodide, water and unconsumed reactants which may be recycled to the reactor to maintain their concentrations in the liquid reaction composition.
EP2060553 discloses a process for the conversion of a carbonaceous feedstock to ethanol wherein the carbonaceous feedstock is first converted to ethanoic acid, which is then hydrogenated and converted into ethanol.
U.S. Pat. No. 4,497,967 discloses an integrated process for the preparation of ethanol from methanol, carbon monoxide and hydrogen feedstock. The process esterifies an acetic anhydride intermediate to form ethyl acetate and/or ethanol.
U.S. Pat. No. 7,351,559 discloses a process for producing ethanol including a combination of biochemical and synthetic conversions results in high yield ethanol production with concurrent production of high value co-products. An acetic acid intermediate is produced from carbohydrates, such as corn, using enzymatic milling and fermentation steps, followed by conversion of the acetic acid into ethanol using esterification and hydrogenation reactions.
As such, the need remains for improvements in the integration of acetic acid production and ethanol production.